Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Some vehicles are equipped with a two speed secondary transmission such that a driver can select a high range and a low range. The high range may be selected for on-road transportation while the low range may be used to provide higher speed ratios for off-road use. In some situations, such as transitioning from on-road to off-road or from off-road to on-road conditions, it is desirable to shift between high and low range while the vehicle is moving, preferably without interrupting the flow of power to the vehicle wheels.